The person/company who has hired you as a contractor would write a letter of employment I guess?
So for example I worked with my company on a contract for two months.Where they employed me on full-time basis and I was salaried.
The only difference was that I had to do the taxes myself for that period.
There is a difference between a contract employee (taxes paid by employer, get a T4) and a self-employed contractor (pay your own taxes, get a T4A). If you are the latter, the business giving you the contract may well write a letter, but I'd be very surprised if they called you an employee (since you aren't, and there are legal ramifications).
Rest I might be wrong but that is how I see it. And I have seen an IT full-time contractor getting through CEC on the same forum as well.
Some people get lucky and squeak through because the VO missed something. Doesn't mean that will happen all the time.